The invention relates, in some embodiments, to the fields of aerosols and fluidics, windowless optical analysis systems, particle detection systems and particle counter systems, in particular flow cells with passive sheath flow and windowless optical detection.
Optical detection of particles in aerosols and fluid suspensions is essential for a wide range of air and gas quality measurements. It is also utilized in many laboratory analytical techniques. The most straightforward particle detection technique involves passage of the aerosol/suspension through a cylindrical flow cell. Such flow cell may possess but is not limited to two opposing transparent windows in its wall. These parallel windows allow the passage of radiation from an external light source, into and out of the flow cell and into a light detector.
The major drawback of this design is that windows made out of any transparent material (e.g. glass, quartz, TiO2) are subject to particle adhesion. Particles from the aerosol or the suspension adhere to the transparent windows and interfere with light passage through the sample. With time, particles may deposit onto proximal optical surfaces, preventing accurate optical detection of particles and accurate particle counting. To overcome this obstacle, flow cells are sometimes modified to enable “windowless optical detection”. Windowless optical detection for fluid suspensions is achieved by having the optical detection system mounted at a small distance away from a nozzle at the end of the flow cell. When the particle suspension is dispensed from the flow cell through the nozzle, the optical detection system probes the particles in the open air.
The major problem with the above-mentioned windowless detection scheme is that once the suspension exits the flow cell, the concentration of the particles changes, and the resulting measurements are inaccurate. Such a scheme also requires complex mechanical design. New methods for accurate windowless detection of particles in aerosols and fluids are required, that will prevent the loss of particles in the open air and that will enable accurate optical detection. New windowless detection methods are required that will allow simplicity of operation.